Effect of Oxygen on Irradiated Males and Females of Drosophila

W IT H IN the last few years much interest has been concentrated on the variations in differential sensitivities to ionizing irradiation which have been found in both plants and animals.These variations may be inherent in that they depend on processes or physiological states in the test object which, while sometimes recognized, can not always be controlled, or they may be induced by different chemical and physical agents.The variations may be in reverse directions: increasing or decreasing the effects of the irradiation.Particular interest has been paid to the protection action of low oxygen concentrations during the period of irradiation.Two hypotheses have been proposed to explain this protective action: (a) At low oxygen concentrations there are induced fewer primary chromosome breaks than in higher oxygen concentrations.(b) There is induced the same rate of primary breaks but low oxygen concentrations favour the rejoining of broken ends.The results of earlier investigations have been interpreted as favouring the first hypothesis (THODAY and READ, 1949;GILES, 1952).Later experiments by SCHWARTZ (1952) with maize have been interpreted in terms of the latter hypothesis.In experiments with Drosophila, BAKER and VON HALLE (1953) have also considered their results as supporting the latter hypothesis.There is, however, a fundamental difference between the original statement of SCHWARTZ and the later statements of BAKER and VON HALLE although both support the »rejoining hypothesis».SCHWARTZ has stated: »Chromosomes broken in the absence of oxygen are more capable of rejoining than chromosomes broken in the presence of oxygen».This means that the rate of breaks which remain unjoined is lowered in absence of oxygen.BAKER and VON HALLE, however, state that »... the breaks induced in N. are more likely to rejoin and to rejoin more quickly than those induced in air.Since the only type of rejoining possible in the sperm is restitution (excluding sister-strand fusion if the chromosome is split) the end result would be fewer breaks available at the time